Cordless telephones have become a staple in today's homes. They offer nearly the portability of a cellular telephone, but cost no more to use than a wireline telephone.
Cordless telephones consist of a base and a handset, each containing a wireless transceiver. The base plugs into a wireline telephone jack and derives its power from a wall outlet. The handset is battery-powered. The base often provides a cradle for the handset to allow the handset to be conveniently stored and recharged.
The number of cordless telephones is and has been rapidly increasing due to user demands for higher mobility and broader general utility. This has resulted in a higher demand for improved performance from the rechargeable batteries to allow the telephones to be operationally available for a greater percentage of the time. Conventional battery charging and monitoring systems are focused on simplicity of operation, regardless of any degenerative impact that the systems may have on the batteries themselves. For example, a conventional battery charging and monitoring system may provide a fixed charging current during the charging period. This fixed charging current is typically maximized to assure that the batteries are charged quickly. Alternatively, a battery may be charged only by a trickle current, which may not damage the battery, but requires an unacceptably long period of time to charge a fully discharged battery.
In spite of these problems, an even more critical problem exists. Batteries quick-charged for faster availability are often at risk of being overcharged. Overcharging a battery usually results in partial, or even permanent, damage to the battery. In addition, overcharging results in a waste of electricity.
To address this problem, prior art battery chargers attempted to detect when the battery was fully charged so they could prevent overcharging and damaging the battery. Designers of such battery chargers noticed that batteries tend to exhibit a small, but detectible, drop in their terminal voltage when they reach a full charge.
Unfortunately, not all batteries exhibit a detectible voltage drop. In fact, some batteries exhibit no drop whatsoever. To complicate matters, battery characteristics change over time. Batteries that may have exhibited detectible drops at one time, now no longer may. These facts kept prior art battery chargers from being able to detect full charges reliably for all batteries that they may be called upon to charge.
Accordingly, what is needed in the art is a battery charger, and related method, that can more reliably determine when a battery is fully charged. In addition, what is needed in the art is a cordless telephone set that incorporates the charger or the method.